This invention relates to wet process chemistry treatment of substrates, e.g., semiconductor wafers and the like, and is more particularly directed to a technique for plating a flat workpiece in a manner that is efficient and also minimizes surface defects. The invention also concerns a technique that facilitates robotic handling of the articles.
Electroplating and electroless plating play significant roles in the production of many rather sophisticated technology products. These techniques have recently evolved for metallization of semiconductor devices. Recently there has been interest in plating techniques to form copper conductors on silicon to increase the power or speed of semiconductor devices. Electroplating and electroless plating may also be used to prepare recordable compact disk blanks. Other related wet-process techniques include etching, stripping and planing.
Several techniques for wet-process treating or coating are described in patent literature.
One recent technique that employs a laminar flow sparger or injection nozzle within the plating bath is described in my recent U.S. Pat. No. 5,597,460, granted Jan. 28, 1997. The means described there achieve an even, laminar flow across the face of the substrate during the plating operation. A backwash technique carries the sludge and particulate impurities away from the article to be plated, and produces a flat plated article of high tolerance, such as a high-density compact disc master or semiconductor wafer. The techniques in that patent improve the flow regime for the plating solution within the tank or cell, as the flow regime is regarded as being crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and uniformity of flow of the fluid as it is contacts and flows across the substrate in the plating cell.
In the plating cell as described in said U.S. Pat. 5,597,460, a plating bath contains the electrolyte or plating solution, and the substrate to be plated is submerged in the solution. A sparger or equivalent injection means introduces the solution into the plating bath and creates a laminar flow of the electrolyte or plating solution across the surface of the substrate to be plated. A circulation system draws off the solution from the anode chamber, together with any entrained particles, and feeds the solution through a microfilter so that all the particles of microscopic size or greater are removed from the plating solution. Then the filtered solution is returned to the sparger and is reintroduced into the plating cell.
The flow regime as described in said U.S. Pat No. 5,597,460 is further improved by the geometry of the well that forms the tank for the plating bath. The well has a cylindrical wall that is coaxial with the axis of the substrate. This arrangement was intended to avoid corners and dead spaces in the plating cell, where either the rotation of the substrate or the flowing movement of the plating solution might otherwise create turbulences.
An increased evenness in plating is achieved by the technique of my U.S. Pat. No. 5,634,564 in which a rotary blade or wiper is positioned in the plating bath.
Electroless plating is favored in many applications, and especially in those where there is no electrically conductive layer that could serve as a cathode. Accordingly, electroless plating is now seen as an economical alternative to sputtering or vacuum deposition. This is especially true for metals that are difficult to deposit using sputtering or plasma techniques. One advantageous approach to electroless plating is disclosed in my earlier U.S. Pat. No. 5,865,894. In that arrangement, a megasonic transducer adjacent the floor of the plating cell applies megasonic energy at a frequency of about 0.2 to 5 MHz to the solution. The frequency can be above 1 MHz, and in some cases above 5 MHz. The megasonic waves distribute the solution evenly on the substrate, and also break up any bubbles or concentrations that may lead to defects in the plated surface.
Megasonic plating techniques can improve the electroplating of silicon wafers. An example of such technique, in which the flow regime is firther improved by imposing a rotary motion, is described in my U.S. Pat. No. 5,865,894. The megasonic transducer and the rotary blade can be incorporated together in a plating cell, as described and illustrated in my U.S. Pat. No. 5,904,827. The techniques described in my U.S. Pat. No. 5,932,077 permit mounting of the substrate and lowering of the substrate into the plating cell to be automated or robotized. Automation and robotization of the insertion, removal, and transport of the workpiece from one process cell to another make it possible to conduct the entire multiple-step plating operation in a clean or super-clean environment. In that technique the carrier for the substrate is disposed on a sealable door for the plating cell. The door opens to a loading position, which is preferably the horizontal position, and closes to a position which preferably holds the substrate vertically in the plating chamber. The door sealably seats onto an opening in a side wall of the cell. For electroplating use, a cathode ring may be disposed at the periphery of the door opening for making electrical contact with the substrate when the door is closed. This arrangement can lend itself to robotization of the plating process, but nevertheless requires the transfer of the substrate from a transfer holder to a platen associated with the plating cell. Moreover, mechanical and fluid handling considerations must be addressed because of the need to move the substrate between horizontal and vertical orientations.
High precision electroplating in the past has required either rotation of the substrate or rotation of a wiper to induce the removal of hydrogen bubbles from the surface to obtain uniform plating free of defects. For this reason the substrate had to be positioned either in a vertical orientation or in a diagonal or slant orientation, rather than horizontal, so that the bubbles would not collect on the surface. Until quite recently, there has been no effective technique for plating, etching, or stripping wafers or other substrates in a horizontal, circuit-side-down, orientation. My now-pending U.S. patent application Ser. No. 09/314,400, filed May 15, 1999, now U.S. Pat. No. 6,217,735, describes a technique for electroplating in which an elongated megasonic transducer produces a ridge of electrolyte that sweeps across the face of the wafer or substrate. The substrate is held horizontal, and with the planar face, i.e., the circuit side of the wafer of other substrate, oriented downwards. The associated electroplating bath employs an elongated, horizontally extending tray that has an open top. An elongated, horizontally extending megasonic transducer is situated at the base of the tray, and an anode extends horizontally above the transducer. A sparger arrangement supplies a flow of a process fluid, such as an electrolyte, into the tray, and the megasonic transducer creates a transverse ridge of the electrolyte or other wet process fluid that projects upwards from the tray. This ridge can contact a substrate passing over the tray. Plating involves applying megasonic energy to the transducer to create the transverse ridge of said electrolyte. Plating current is applied between the anode and the substrate. The substrate is oriented horizontally and face down so it is in position to contact the transverse ridge of electrolyte, and then either the substrate holder or the tray is moved in the direction across the ridge to effect relative motion as between substrate and tray so that the ridge sweeps across the face of the substrate. A rinser may be positioned alongside the electrolyte tray for rinsing the substrate after the plating operation. The rinser may also involve employ a megasonic transducer to create a ridge of rinse solution. In the technique of patent application Ser. No. 09/314,400, the anode is preferably a transverse metal rod or similar conductive member that extends parallel to the transducer and just above it within the electrolyte. Alternatively, the anode can be a stainless steel surface incorporated as a lens of the transducer. The holder for the wafer or other substrate may be a heated or unheated chuck. The megasonic energy also heats the substrate where the ridge contacts it, and in some cases may partially or fully heat-treat the metallization.
Preferably, the sparger arrangement creates a non-turbulent flow of the electrolyte that emanates from each side of the megasonic transducer, such as rows of openings, one row disposed along one side of the transducer, and one row disposed along the other side.
In this arrangement, the chuck or tray has to move back and forth mechanically. This requires a significant amount of mechanical drive equipment. The horizontal drive can be expensive and complex, and require significant service and repair. In addition, the moving parts can be a source of particulate contaminants that may fall into the plating bath in the tray. Also, the speed and direction of sweep are limited, and so there is not much flexibility afforded in the pattern of the ridge or how it moves across the substrate.
Accordingly, it would be advantageous to provide wet-process equipment in which a ridge can be generated and controlled for plating a substrate, but where the mechanical moving parts are kept to a minimum, and where there is increased flexibility in speed, shape, and direction of motion of the ridge across the substrate.
At this time, there are available submersible megasonic arrays, which contain a number of transducers arranged in a grid, and which are actuated by microprocessor control. These are typically used for cleaning applications where a number of wafers are carried in a cassette or "boat" and are submerged in the cleaning or rinse solution and are all cleaned at the same time. To date, no one has used this type of megasonic transducer arrangement to create a controlled ridge that extends above the surface of the liquid.